The present invention relates to a color cathode-ray tube and, more particularly, to stabilizing the inner wall potential in the neck of the cathode ray tube.
A color cathode-ray tube generally comprises an envelope made up of a panel, a funnel, and a neck, an inner conductive film formed from the inner wall of the funnel to the inner wall of the neck, and an electron gun assembly accommodated in the neck and having cathodes located at the end portion in the neck and a plurality of grids arrayed from the cathode side in order.
The most popular electron gun assembly is a so-called in-line electron gun assembly in which three electron guns align in line.
The electron gun comprises a beam generation portion called a triod portion constituted by a cathode, a first electrode, and a second electrode, and a main lens portion for focusing an electron beam on a screen. These electrodes are arranged at predetermined intervals (electrode gaps). The electron gun is sealed in a cylindrical neck portion having a diameter of, e.g., 20 to 40 mm in the rear portion of the cathode-ray tube.
An external voltage of about 110 volts is applied to the cathode of the electrodes of the triod portion through stem pins extending through the terminal portion of the neck to allow connection between the interior and the exterior of the tube. An external voltage of 0V is applied to the first electrode, and an external voltage of several hundred volts is applied to the second electrode.
An electron beam generated by the cathode passes through the beam transmission holes of the first and second electrodes and reaches the main lens. The electron beam is finally focused on the screen by the main lens.
The main lens is constituted by at least two electrodes. One of these electrodes is a final acceleration electrode connected to a high anode voltage through the inner conductive film formed on the inner surface of the neck. At least another electrode of the main lens is a convergence electrode to which a voltage 20 to 40% of the high anode voltage is applied through the stem pins.
The beam transmission holes of the final acceleration electrode and the convergence electrode generally oppose each other to have an interval of about 1 mm. The voltages described above are applied to these opposing beam transmission holes to form the main lens, thereby focusing the beam on the screen.
These electrodes are fixed and supported by insulating supports made up of glass or the like.
The projecting portions (straps) of the respective electrodes are embedded in the insulating supports to fix and support the respective electrodes. These straps are generally formed on the center gun of the three electron guns.
In the color cathode-ray tube having the above electron gun assembly, even the inner surface portion not coated with the inner conductive film is gradually charged up to the high anode voltage of the inner conductive film by the inner conductive film coated on the inner surface of the neck. The inner surface portion not coated with the inner conductive film stabilizes at a given potential within several ten minutes to several hours.
When an electron beam is emitted by the cathode, scattered electrons leaking from the electrode gap or the like without passing through each electrode beam transmission hole collide against the inner surface of the neck to emit secondary electrons from the inner surface of the neck, thereby undesirably varying the neck potential.
The neck potential penetrates through each gap between the electrodes constituting the electron gun, and the trajectory of the electron beam changes due to the variations in neck potential.
This penetration of the neck potential occurs from the entire inner surface of the neck. However, the center gun assembly is located at nearly the center of the neck; the Coulomb forces generated by the neck potential almost balance each other in this assembly, and the trajectory of the center beam is hardly affected by the neck potential. As the inner surface of the neck is located symmetrically in the vertical direction, the trajectory of each side beam in the vertical direction changes little for the same reason in a direction perpendicular to the array direction of the three electron guns (this direction is also referred to as the vertical direction hereinafter).
Of the changes in beam trajectories by the penetration of the neck potential, a change in trajectory of the side beam in the horizontal direction which has an asymmetrical relationship with the inner surface of the neck poses a practical problem. A so-called convergence drift occurs due to this change, resulting in color misregistration.
The change in beam trajectory by the penetration of the neck potential is the largest in the main lens portion having a largest electrode gap and closest to the internal conductive film.
A means for removing the convergence drift generated by the neck potential is to reduce the electrode gap to make it difficult for the neck potential to penetrate. However, the electrode gap cannot be excessively reduced in consideration of the withstanding voltage characteristics between the electrodes. The above means is not an effective means.
Another means is to increase the outer diameter of each electrode constituting the electron gun and make it difficult for the neck potential to penetrate. However, the inner diameters of the neck and the insulating supports which support the electrodes are limited. The outer diameter of each electrode cannot be simply increased, and a sufficient effect cannot be obtained.
Still another means is a technique proposed by Jpn. Pat. Appln. KOKAI Publication No. 5-205660 to form a high-resistance film connected to an inner conductive film, i.e., a film in which a conductive material is dispersed in an insulator film serving as a base material, thereby stabilizing the neck potential. The resistance of the high-resistance film must be extremely high in consideration of the withstanding voltage characteristics and must fall within the range of, e.g., about 10.sup.10 to about 10.sup.14 .OMEGA.. To increase the resistance, the amount of insulator in the high-resistance film must be increased. Such increase in the amount of insulator in the high-resistance film promotes emission of secondary electrons from the scattered electrons. The convergence drift cannot be perfectly eliminated.